1. Field of the Invention
The present invention relates to an oxytetramethylene glycol copolymer and a method for producing the same. More particularly, the present invention is concerned with an oxytetramethylene glycol copolymer obtained by copolymerizing tetrahydrofuran and neopentyl glycol, wherein the oxytetramethylene glycol copolymer has a specific number average molecular weight, a specific molecular weight distribution and a specific neopentyl glycol copolymerization ratio. The present invention is also concerned with a method for producing such an oxytetramethylene glycol copolymer. The oxytetramethylene glycol copolymer of the present invention exhibits improved low temperature properties due, for example, to low melting point and low glass transition temperature. By virtue of these improved properties, the oxytetramethylene glycol copolymer of the present invention can be advantageously used as a raw material for an elastic fiber and the like. Further, the present invention is concerned with a method for purifying an oxytetramethylene glycol copolymer, obtained by copolymerizing tetrahydrofuran and a diol by the method of the present invention or any conventional method, from a copolymerization reaction mixture comprising an oxytetramethylene glycol copolymer and an unreacted diol, wherein the unreacted diol is distilled off in the presence of fresh tetrahydrofuran. By the use of the purification method of the present invention, it becomes possible to not only purify the copolymer without causing the clogging of a condensation tube and a conduit by the solidification of the diol, but also recover a recyclable diol.
2. Prior Art
Recently, an oxytetramethylene glycol copolymer obtained by copolymerizing tetrahydrofuran (THF) and a diol (such as neopentyl glycol) and an oxytetramethylene glycol copolymer obtained by copolymerizing THF and 3-methyltetrahydrofuran have been drawing attention. An oxytetramethylene glycol copolymer has a lower melting point than an oxytetramethylene glycol homopolymer (that is, polyoxytetramethylene glycol (PTMG)), and an elastic product produced using an oxytetramethylene glycol copolymer as a raw material exhibits remarkably improved elongation, hysteresis loss and low temperature properties, as compared to those of an elastic product produced using PTMG as a raw material. For example, at a temperature below the ice point, a conventional polyurethane urea elastic fiber produced using PTMG exhibits no instantaneous elastic recovery, but a polyurethane urea elastic fiber produced using an oxytetramethylene glycol copolymer exhibits an instantaneous elastic recovery which is substantially the same as observed at room temperature.
An oxytetramethylene glycol copolymer can be easily synthesized by using a heteropolyacid as a polymerization catalyst. For example, each of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 60-203633 (corresponding to European Patent Publication No. 158,229B), Unexamined Japanese Patent Application Laid-Open Specification No. Sho 61-120830 (corresponding to European Patent Publication No. 158,229B) and Unexamined Japanese Patent Application Laid-Open Specification No. Sho 61-123630 discloses an oxytetramethylene glycol copolymer obtained by copolymerizing a diol and THF in the presence of a heteropolyacid in either a batchwise manner or a continuous manner. In the technique of each of the above-mentioned patent documents, diol molecules are incorporated, into the oxytetraethylene glycol copolymer chains, mainly as a terminator for the living cationic polymerization of THF. Therefore, most of the diol molecules which are incorporated into the polymer chains are present at the terminals of the polymer chains and the average diol copolymerization ratio is approximately one molecule. According to the reaction modes disclosed in the abovementioned patent documents, the average number of diol molecules that can be incorporated into a copolymer molecule (that is, average diol copolymerization ratio) is approximately 1 molecule and, thus, there is a limitation in the melting point-lowering effect of the copolymerization of a diol (copolymerization effect). In addition, in the above-mentioned patent documents, removal of water during the copolymerization reaction is disclosed as a method for increasing the copolymerization ratio of a diol (specifically, to achieve a copolymerization ratio as high as approximately 10 to 35 moles). In this method, when a water removal step is conducted in addition to the standard copolymerization step, there is a limitation that the number of water molecules coordinated to a heteropolyacid used as the polymerization catalyst must be in the range of from 0.1 to 15. Especially when a copolymerization reaction is conducted using a heteropolyacid having a water coordination number as high as 6 to 15, the reaction rate becomes markedly lowered. As a consequence, the time of polymerization becomes very long and only an oxytetramethylene glycol copolymer having a broad molecular weight distribution can be produced using such a heteropolyacid. An oxytetramethylene glycol copolymer having a broad molecular weight distribution has a problem in that the glass transition temperature is high.
Further, working examples of Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 6-87951, Hei 9-291147, Hei 10-87811, Hei 10-87812, Hei 10-87813 and the like disclose methods for incorporating 1 to 5 moles of neopentyl glycol (NPG) into 1 molecule of an oxytetramethylene glycol copolymer. Specifically, each of these patent documents discloses a method which comprises polymerizing NPG and THF in the presence of a heteropolyacid catalyst in a batchwise manner, wherein the polymerization reaction is performed while removing the by-produced water from the reaction system by distillation. In a batchwise reaction, even when the NPG concentration is high at the initial stage of the reaction, the NPG concentration of the reaction system becomes markedly lowered at the final stage of the polymerization reaction. Therefore, low molecular weigh copolymers having high NPG copolymerization ratio are likely to be produced at the initial stage of the polymerization reaction. The thus produced low molecular weight copolymers having high NPG copolymerization ratio are likely to polymerize and mature into high molecular weight copolymers, thereby producing a high molecular weight copolymer having a relatively high copolymerization ratio. Due to the presence of such a high molecular weight copolymer having a high copolymerization ratio, the glass transition temperature of the copolymers as a whole is caused to become high, thereby rendering it difficult to produce an oxytetramethylene glycol copolymer having a low glass transition temperature.
As mentioned above, an oxytetramethylene glycol copolymer is produced by the copolymerization reaction of THF and a diol in the presence of a heteropolyacid as a polymerization catalyst. Several hundred ppm to several percent of an unreacted diol usually remains in the reaction mixture obtained after the copolymerization reaction, and when an elastic product (e.g., an elastic fiber, such as urethane urea) is produced using an oxytetramethylene glycol copolymer containing residual unreacted diol, the elastic product is incapable of exhibiting the intended properties because the residual diol (such as unreacted NPG) do not function as a soft segment. For solving this problem, it was attempted to perform the copolymerization reaction under conditions which enable a complete consumption of the diol or, alternatively, to remove unreacted diol from the copolymer containing the unreacted diol.
For achieving the complete consumption of the diol during the copolymerization reaction, it is necessary to conduct the copolymerization reaction in a batchwise manner at a high reaction temperature while removing the by-produced condensation water thoroughly from the reaction system so as to shift the reaction equilibrium. When the residual amount of the unreacted diol is lowered by this method to less than 100 ppm, adverse side reactions, such as liberation of terminal hydroxyl groups from the diol as well as from the produced oxytetramethylene glycol copolymer, are likely to occur due to the action of heat thus leading to an occurrence of discoloration of and lowering of the quality of the oxytetramethylene glycol copolymer. In addition, this method is only applicable to a batchwise reaction and, thus, as mentioned-above, an oxytetramethylene glycol copolymer having low glass transition temperature cannot be produced by this method.
As other methods for removing a diol from a copolymer containing an unreacted diol, there can be mentioned a method in which a diol is selectively adsorption-removed by means of an adsorbent (Unexamined Japanese Patent Application Laid-Open Specification No. Hei 9-291147), a method in which a diol is removed by extraction (Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-87813) and a method in which a diol is removed by vacuum distillation (Unexamined Japanese Patent Application Laid-Open Specification No. Hei 1-92221 (corresponding to European Patent Specification No. 305,853B)).
In the method in which a diol is removed by means of an adsorbent, the adsorbing removal ratio and breakthrough time vary depending on the type of the adsorbent used, the type and amount of the diol being adsorbed, and the like. Therefore, the type and amount of the adsorbent must be changed in accordance with the change in the conditions employed for producing an oxytetramethylene glycol copolymer. In addition, not only the unreacted diol, but also a large amount of low molecular weight oxytetramethylene glycol copolymers is adsorbed on the adsorbent, and the loss of the low molecular weight oxytetramethylene glycol copolymers becomes large. Further, for recycling the adsorbed unreacted diol, the diol must be desorbed from the adsorbent by using a solvent, such as THF. In this case, the desorption ratio also varies depending on the type and amount of the diol adsorbed on the adsorbent, and the amount of the solvent necessary for desorption varies drastically. The desorption of the adsorbed low molecular weight oxytetramethylene glycol copolymers is also accompanied by the drastic variation in the amount of the desorbed low molecular weight oxytetramethylene glycol copolymers. After the desorption of the low molecular weight oxytetramethylene glycol copolymers by a solvent, such as THF, an additional step is necessary for determining the amount of the low molecular weight oxytetramethylene glycol copolymers contained in the desorbate and specifically adjusting the copolymer concentration thereof before using the desorbed low molecular weight oxytetramethylene glycol copolymers. Therefore, this method has several problems for use in an industrial process. Further, the problems accompanying the desorption of the low molecular weight oxytramethylene glycol copolymers are known to become more difficult when the amount of the residual diol contained in the oxytetramethylene glycol copolymer decreases.
With respect to a method in which a diol is removed by extraction, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-87813 discloses a method in which a diol is extraction-removed with water. In this method, the amount of the extraction agent (water) varies depending on the type and amount of a diol remaining in the copolymer. In addition, a polymer loss is likely to occur because an oxytetramethylene glycol copolymer occurs simultaneously with extraction-removal of a diol. In this method, by using the extraction agent in a large amount, the amount of the residual diol in the oxytetramethylene glycol copolymer can be reduced to a level which is not more than a predetermined value, but the polymer loss accompanying the extraction becomes increased. For recycling the diol extracted by this method, water used as the extraction agent must be removed by distillation. As a result, the purification process becomes disadvantageously complicated for a commercial scale production of an oxytetramethylene glycol copolymer.
Unexamined Japanese Patent Application Laid-Open Specification No. Hei 1-92221 discloses a method for removing a diol by vacuum distillation. Specifically, a diol is removed under conditions wherein the pressure is not more than 0.3 mbar, and the temperature is 200 to 260° C. Under such distillation conditions, an oxytetramethylene glycol copolymer becomes deteriorated. Further, since this method is a method for separating low molecular weight components from PTMG, when this method is used for removing a diol which is generally solidified at room temperature, the diol distilled off from the distillation apparatus becomes solidified during the subsequent cooling process, thereby causing the clogging of a condensation tube, a conduit and other components which are disposed in the vicinity of a vacuum pump. Therefore, this method is substantially inapplicable to a process in which a diol is continuously distilled off.